Lignin is one of the three basic components of lignocellulosic biomass, the other two being cellulose and hemicellulose. On a dry basis, lignin is about 17 wt %-33 wt % of the biomass and about 40% of its energy (27 wt % to 33 wt % in softwoods, 18 wt % to 25 wt % in hardwoods, and 17 wt % to 24 wt % in grasses; Bozell, et al., Top Value Added Chemicals from Biomass—Volume II, PNNL—16983, 2007), whereas cellulose and hemicellulose together are about 60 wt %-85 wt % of the biomass. Lignin is a natural, amorphous, cross-linked three-dimensional polyphenolic compound. Although the exact structure of lignin is complex and changes depending on which biomass it is part of, in general terms, lignin contains phenylpropenyl (C9) branched units connected to each other with either carbon-carbon or carbon-oxygen (ether) bonds of the type β-O-4, 5-5, β-5, β-1, α-O-4,4-O-5, β-β, etc. Lignin is the only biomass polymer that contains aromatic units, and it is estimated that about 40 wt % of lignin is aromatic. Lignin is made naturally by enzymatic polymerization of coniferyl alcohol, sinapyl alcohol, and para-coumaryl alcohol. These alcohols are essentially the monomeric units in lignin. Coniferyl alcohol (4-(3-hydroxy-1-propenyl)-2-methoxyphenol) has one —OH (hydroxy) group, one —OCH3 (methoxy) group, and one —CH═CHCH2OH (hydroxypropenyl) group; sinapyl alcohol (4-(3-hydroxyprop-1-enyl)-2,6-dimethoxyphenol) has one —OH group, two —OCH3 groups, and one —CH═CHCH2OH group; and para-coumaryl alcohol (4-(3-hydroxy-1-propenyl)phenol)) has one —OH group and one —CH═CHCH2OH group. Thus, the monomeric units in lignin have phenolic hydroxy, hydroxypropenyl, and methoxy pendant groups from an aromatic ring, with the methoxy groups being the most abundant per 100 C9 units (e.g. about 95 methoxy pendant groups per 100 C9 units in softwood lignin).
The global production of lignin today is about 1 million tons, and it comes in various forms as a by-product of pulp and paper operations. One form of lignin is kraft lignin, which is produced from the sulfate pulping process, has a molecular weight of 2,000 to 3,000 g/mol, and has an average molecular weight of the monomeric unit of 180 g/mol. Another form of lignin is lignosulfonate, which is produced from the sulfite pulping process, has a molecular weight of 20,000 to 50,000 g/mol, and has an average molecular weight of the monomeric unit of 215 to 254 g/mol. A third form of lignin is organosolv lignin, which is produced from the alcohol pulping process, has a molecular weight of less than 1,000 g/mol, and an average molecular weight of the monomeric unit of 188 g/mol (Lebo et al., Kirk-Othmer Encyclopedia of Chemical Technology, Online Edition, J. Wiley & Sons, 2001). Main uses of lignin today are for power generation, detergents, dispersants, additives, raw materials for vanillin, humic acid, etc., dust suppression agents, etc.
Lignin is expected to be produced in much larger quantities in the future as many companies are commercializing cellulosic sugars and ethanol, and lignin becomes a by-product of these operations. For example, in a recent study, the US DoE suggested that 1.3 billion tons of biomass is available annually in the US for biofuels and biomaterials. This amount of biomass could yield about 400 million tons of lignin annually. Converting this large amount of lignin to high-value and/or high-volume chemicals and fuels (e.g. phenol, benzene, toluene, xylenes, etc.), which today are made from petroleum or natural gas sources, could help the environmental profile of these chemicals, as well as could lower their production cost. Additionally, this use of lignin could help lower the final price of cellulosic sugars and ethanol, which will be co-produced in a bio-refinery type of operation.
To convert lignin or bio-oils from a fast pyrolysis process, to chemicals such as, benzene, toluene, and xylenes, one has to remove oxygen and add hydrogen, i.e., to perform a hydrotreating reaction. In the petroleum industry, catalysts have been used for a similar hydrotreating operation, called hydrotreating, where sulfur or nitrogen are removed via hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), respectively. Similar catalysts have been proposed for hydrotreating of lignin compounds or model monomers. However, these HDS catalysts are not stable in water or alcohol solvents, showed poor conversion in hydrotreating environments, required co-feeding of H2S to maintain their activity, and exhibited high costs (Deutsch, K. L., and B. H. Shanks, Applied Catalysis A: General, 447-448, 144-150 (2012)).
Accordingly, there is a need for processes and catalysts for the conversion of methoxylated aromatic compounds, including depolymerized lignin compounds, to simple aromatic compounds with high yield, selectivity, and efficiency (i.e., short residence time), and high longevity catalysts.